the reference folder must be contained within the module. In the case of the example provided with the module folder is called "Images" and contains two subfolders named: "amster" and "berlin". In this example you can be entered as a parameter: "images" and then it will show all images contained in subfolders and will show the buttons (all, amster, berlin), but if specified as a parameter: "images / amster" will be shown only the images in this subfolder and buttons will not be shown. This second case is illustrated on the page in the second example.